Semiconductor devices with trench gate structures in a semiconductor body with hexagonal crystal lattice

ABSTRACT

A semiconductor device includes trench gate structures in a semiconductor body with hexagonal crystal lattice. A mean surface plane of a first surface is tilted to a &lt;1-100&gt; crystal direction by an off-axis angle, wherein an absolute value of the off-axis angle is in a range from 2 degree to 12 degree. The trench gate structures extend oriented along the &lt;1-100&gt; crystal direction. Portions of the semiconductor body between neighboring trench gate structures form transistor mesas. Sidewalls of the transistor mesas deviate from a normal to the mean surface plane by not more than 5 degree.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Serial No.15/053,117, filed Feb. 25, 2016, and entitled “Semiconductor Device withTrench Gate Structures in a Semiconductor Body with Hexagonal CrystalLattice,” which claims priority to German Application Serial No.102015103067 filed Mar. 3, 2015, and entitled “Semiconductor Device withTrench Gate Structures in a Semiconductor Body with Hexagonal CrystalLattice.”

BACKGROUND

Power semiconductor switches withstand a blocking voltage of severalhundred Volts at high current rating. Typically, a load current througha power semiconductor switch flows in a vertical direction between thetwo principal planes of a semiconductor body. Gate electrodes forcontrolling the load current may be formed in trench gate structuresextending from one of the principal planes into the semiconductor body.On the other hand, semiconductor materials with hexagonal crystallattice such as silicon carbide and gallium nitride have seen increasinginterest in the use as base material for electronic circuits.

It is desirable to improve the device characteristics and devicereliability of semiconductor devices with hexagonal crystal lattice.

SUMMARY

According to an embodiment a semiconductor device includes trench gatestructures in a semiconductor body with hexagonal crystal lattice. Amean surface plane of a first surface is tilted to a <1-100> crystaldirection by an off-axis angle, the absolute value of which is in arange from 2 degree to 12 degree. The trench gate structures extendoriented along the <1-100> crystal direction. Portions of thesemiconductor body between neighboring trench gate structures formtransistor mesas. Sidewalls of the transistor mesas deviate from anormal to the mean surface plane by not more than 5 degree.

Those skilled in the art will recognize additional features andadvantages upon reading the following detailed description and onviewing the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification. The drawings illustrate the embodiments ofthe present invention and together with the description explainprinciples of the invention. Other embodiments of the invention andintended advantages will be readily appreciated as they become betterunderstood by reference to the following detailed description.

FIG. 1A is a schematic plan view of a surface portion of a semiconductorbody of a semiconductor device according to an embodiment, wherein thesemiconductor body has a hexagonal crystal lattice and includes a trenchgate structure.

FIG. 1B is a schematic vertical cross-sectional view of thesemiconductor device portion of FIG. 1A along line B-B intersecting alongitudinal axis of the trench gate structure.

FIG. 1C is a schematic vertical cross-sectional view of thesemiconductor device portion of FIG. 1A along line C-C through atransistor mesa and parallel to a longitudinal axis of the trench gatestructure.

FIG. 1D is a schematic vertical cross-sectional view of thesemiconductor device portion of FIG. 1A along line D-D along thelongitudinal axis of the trench gate structure.

FIG. 2 schematically shows a lattice cell of a hexagonal crystal latticefor discussing effects of the embodiments.

FIG. 3A is a schematic perspective view illustrating the crystalorientations in a lattice cell of a hexagonal crystal lattice with the<0001> crystal direction tilted with respect to a mean surface plane indirection of the <11-20> crystal planes for discussing effects of theembodiments.

FIG. 3B is a schematic cross-sectional view of a portion of asemiconductor body including a trench gate structure, wherein a meansurface plane of the semiconductor body is tilted to the <11-20> crystaldirection.

FIG. 4A is a schematic perspective view illustrating the crystalorientations in a lattice cell of a hexagonal crystal lattice with the<0001> crystal direction tilted with respect to a mean surface plane indirection of the <1-100> crystal planes for discussing effects of theembodiments.

FIG. 4B is a schematic vertical cross-sectional view of a portion of asemiconductor body including a trench gate structure, wherein a meansurface plane of the semiconductor body is tilted to the <1-100> crystaldirection.

FIG. 4C is a schematic vertical cross-sectional view of thesemiconductor device portion of FIG. 4B along line C-C.

FIG. 5A is a schematic horizontal cross-sectional view of a portion of asemiconductor device according to an embodiment related to deep mesacontact structures.

FIG. 5B is a schematic vertical cross-sectional view of thesemiconductor device portion of FIG. 5A along line B-B.

FIG. 6A is a schematic horizontal cross-sectional view of a portion of asemiconductor device according to an embodiment related to deep trenchcontact structures extending through the trench gate structures.

FIG. 6B is a schematic vertical cross-sectional view of thesemiconductor device portion of FIG. 6A along line B-B.

FIG. 7A is a schematic horizontal cross-sectional view of a portion of asemiconductor device according to an embodiment related to transistorand diode mesas alternating along a direction parallel to longitudinalaxes of the trench gate structures.

FIG. 7B is a schematic cross-sectional view of the semiconductor deviceportion of FIG. 7A along line B-B orthogonal to trench gate structures.

FIG. 7C is a schematic cross-sectional view of the semiconductor deviceportion of FIG. 7A along line C-C parallel to the trench gate structuresand through diode and transistor mesas.

FIG. 7D is a schematic vertical cross-sectional view of thesemiconductor device portion of FIG. 7A along line D-D parallel to andthrough trench gate structures.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings, which form a part hereof and in which are shownby way of illustrations specific embodiments in which the invention maybe practiced. Other embodiments may be utilized and structural orlogical changes may be made without departing from the scope of thepresent invention. For example, features illustrated or described forone embodiment can be used on or in conjunction with other embodimentsto yield yet a further embodiment. It is intended that the presentinvention includes such modifications and variations. The examples aredescribed using specific language, which should not be construed aslimiting the scope of the appending claims. The drawings are not scaledand are for illustrative purposes only. For clarity, the same or similarelements have been designated by corresponding references in thedifferent drawings if not stated otherwise.

The terms “having”, “containing”, “including”, “comprising” and the likeare open, and the terms indicate the presence of stated structures,elements or features but do not preclude additional elements orfeatures. The articles “a”, “an” and “the” may include the plural aswell as the singular, unless the context clearly indicates otherwise.

The term “electrically connected” describes a permanent low-ohmicconnection between electrically connected elements, for example a directcontact between the concerned elements or a low-ohmic connection via ametal and/or highly doped semiconductor. The term “electrically coupled”includes that one or more intervening element(s) adapted for signaltransmission may be provided between the electrically coupled elements,for example elements that are controllable to temporarily provide alow-ohmic connection in a first state and a high-ohmic electricdecoupling in a second state.

The Figures illustrate relative doping concentrations by indicating “−”or “+” next to the doping type “n” or “p”. For example, “n⁻” means adoping concentration which is lower than the doping concentration of an“n”-doping region while an “n⁺”-doped region has a higher dopingconcentration than an “n”-doped region. Doped regions of the samerelative doping concentration do not necessarily have the same absolutedoping concentration. For example, two different “n”-doped regions mayhave the same or different absolute doping concentrations.

FIGS. 1A to 1D refer to a semiconductor device 500 including transistorcells TC. The semiconductor device 500 may be or may include an IGFET(insulated gate field effect transistor), for example an MOSFET (metaloxide semiconductor FET) in the usual meaning including FETs with metalgates as well as FETs with non-metal gates, an IGBT (insulated gatebipolar transistor), or an MCD (MOS controlled diode), by way ofexample.

The semiconductor device 500 is based on a semiconductor body 100 fromcrystalline semiconductor material with a hexagonal crystal lattice,such as silicon carbide (SiC) or an appropriate A_(III)B_(v)semiconductor, e.g., gallium nitride (GaN). According to an embodiment,the semiconductor material is silicon carbide of the 4H polytype(4H—SiC).

At a front side the semiconductor body 100 has a first surface 101,which may be planar or which may include coplanar surface sections,wherein the planar first surface 101 or the coplanar surface sectionsmay coincide with a main crystal plane or may be tilted to a maincrystal plane by an off axis angle α, which absolute value may be atleast 2 degree and at most 12 degree, e.g., about 4 degree. According toother embodiments, the first surface 101 is a serrated or “staggered”surface with parallel first surface sections 101 a shifted to each otherand tilted to a mean surface plane 101 x, which may be given by theleast squares mean plane. Second surface sections 101 b, which aretilted to the first surface sections, connect the first surface sections101 a. A cross-sectional line of the serrated first surface 101 issaw-toothed. On the back, an opposite second surface 102 may extendparallel or tilted to the mean surface plane 101 x or may includeparallel surface sections parallel to the first and second surfacesections 101 a, 101 b of the first surface 101.

A distance between the first surface 101 at the front side and thesecond surface 102 on the back is related to a nominal blocking voltagethe semiconductor device 500 is specified for. Typically, thesemiconductor body 100 includes a first vertical portion thataccommodates the applied electric field in the blocking state, whereinthe thickness of the first portion is proportional to the nominalblocking voltage and defines the electric field breakdown strength,whereas the thickness of further vertical portions, e.g., a substrateportion, is not related to the nominal blocking voltage.

A total thickness of the semiconductor body 100 between the first andsecond surfaces 101, 102 may be in the range of several μm to severalhundred μm. The normal to the mean surface plane 101 x defines avertical direction. Directions parallel to the first surface 101 arehorizontal directions.

The transistor cells TC are formed along trench gate structures 150. Alongitudinal extension of the trench gate structures 150 along a firsthorizontal direction may be greater than a width along a secondhorizontal direction orthogonal to the first horizontal direction. Thetrench gate structures 150 may be long stripes extending from one sideof a transistor cell area to an opposite side. According to otherembodiments a plurality of separated trench gate structures 150 may bearranged along a line extending from one side of the transistor cellarea to the opposite side, or the trench gate structures 150 may form agrid with portions of the semiconductor body 100 formed in the meshes ofthe grid.

The trench gate structures 150 may be equally spaced, may have equalwidth, and may form a regular pattern, wherein a pitch(center-to-center) of the trench gate structures 150 may be in a rangefrom 1 μm to 10 μm, e.g., from 2 μm to 5 μm. A vertical extension of thetrench gate structures 150 may be in a range from 0.3 μm to 5 μm, e.g.,in a range from 0.5 μm to 2 μm.

The trench gate structures 150 include a conductive gate electrode 155which may include or consist of a heavily doped polycrystalline siliconlayer or a metal-containing layer. The trench gate structures 150further include a gate dielectric 151 separating the gate electrode 155from the semiconductor body 100. The gate dielectric 151 may include orconsist of a semiconductor dielectric, for example thermally grown ordeposited semiconductor oxide, e.g., silicon oxide, a semiconductornitride, for example deposited or thermally grown silicon nitride, asemiconductor oxynitride, for example silicon oxynitride, any otherdeposited dielectric material or any combination thereof. The gatedielectric 151 may be formed for a threshold voltage of the transistorcells TC in a range from 1.5 V to 6 V. In end portions of the trenchgate structures 150 the gate dielectric 151 may be thicker than inactive portions of the transistor cells 170 to improve devicereliability. According to an embodiment, the end portions may becompletely filled with a dielectric material such as silicon oxide.

All or some of the mesa portions of the semiconductor body 100 betweenneighboring trench gate structures 150 form transistor mesas 170 thatinclude semiconducting portions of the transistor cells TC. The mesaportions may also include diode mesas including at least portions of abody diode.

The transistor mesas 170 include source zones 110 that are oriented tothe front side and that may directly adjoin the first surface 101. Ineach transistor mesa 170 one or two source zones 110 directly adjoinboth neighboring trench gate structures 150. For example, the transistormesa 170 may include two separated source zones 110, which extend onopposite sides of an intermediate mesa contact structure between themesa contact structure and one of the neighboring trench gate structures150, respectively. According to another embodiment, the source zones 110may extend from one of the trench gate structures 150 adjoining theconcerned transistor mesa 170 to the other, opposite trench gatestructure 150.

The transistor mesas 170 further include body zones 115 that separatethe source zones 110 from a drift structure 120, wherein the body zones115 form first pn junctions pnl with the drift structure 120 and secondpn junctions pn2 with the source zones 110. A vertical extension of thebody zones 115 corresponds to a channel length of the transistor cellsTC and may be in a range from 0.2 μm to 1.5 μm.

Each body zone 115 may extend from one of the trench gate structures 150adjoining the concerned transistor mesa 170 to the other, oppositetrench gate structure 150, or two body zones 115 may extend on oppositesides of an intermediate mesa contact structure between the mesa contactstructure and one of the neighboring trench gate structures 150,respectively. Portions of the body zones 115 are capacitively coupled tothe gate electrode 155 through the gate dielectric 151.

Both the first pn junctions pnl and the second pn junction pn2 mayextend over the whole width of the transistor mesa 170 between the twotrench gate structures 150 sandwiching the concerned transistor mesa170. Both the source zones 110 and the body zones 115 are electricallyconnected to a first load electrode 310 at the front side.

The semiconductor body 100 may further include diode regions 116 formingthird pn junctions pn3 with a drift structure 120. The diode regions 116are electrically connected or coupled to the first load electrode 310and may vertically overlap with the trench gate structures 150 such thatportions of the diode regions 116 are formed in the vertical projectionof the trench gate structures 150. A distance between opposing edges ofneighboring diode regions 116 may be in a range from 2 μm to 3 μm, byway of example.

The drift structure 120 is oriented to the back, may directly adjoin thesecond surface 102 and may be electrically connected or coupled to asecond load electrode 320 through an ohmic contact or a further pnjunction. The drift structure 120 may include a lightly doped drift zone121 that may form the first and third pn junctions pn1, pn3 as well as aheavily doped contact layer 129 between the drift zone 121 and thesecond surface 102. The net dopant concentration in the drift zone 121may be in a range from 1E14 cm⁻³ to 3E16 cm⁻³ in case the semiconductorbody 100 is formed from silicon carbide.

A mean dopant concentration in the contact layer 129 is sufficientlyhigh to ensure an ohmic contact with a second load electrode 320 thatdirectly adjoins the second surface 102. In case the semiconductordevice 500 is a semiconductor diode or an IGFET, the contact layer 129has the same conductivity type as the drift zone 121. In case thesemiconductor device 500 is an IGBT, the contact layer 129 has thecomplementary conductivity type of the drift zone 121 or includes zonesof the complementary conductivity type.

Each of the first and second load electrodes 310, 320 may consist of orcontain, as main constituent(s), aluminum (Al), copper (Cu), or alloysof aluminum or copper such as AlSi, AlCu or AlSiCu. According to otherembodiments, at least one of the first and second load electrodes 310,320 may contain, as main constituent(s), nickel (Ni), titanium (Ti),tungsten (W), tantalum (Ta), Vanadium (V), silver (Ag), gold (Au), tin(Sn), platinum (Pt), and/or palladium (Pd). One of the first and secondload electrodes 310, 320 or both may include two or more sub-layers,wherein each sub-layer contains one or more of Ni, Ti, V, Ag, Au, W, Sn,Pt, and Pd as main constituent(s), e.g., a silicide, a nitride and/or analloy.

The first load electrode 310 may form or may be electrically connectedor coupled to a first load terminal L1, which may be an anode terminalof an MCD, a source terminal of an IGFET or an emitter terminal of anIGBT. The second load electrode 320 may form or may be electricallyconnected or coupled to a second load terminal L2, which may be acathode terminal of an MCD, a drain terminal of an IGFET or a collectorterminal of an IGBT.

According to an embodiment, the transistor cells TC are re-channel FETcells with p-doped body zones 115 and n-doped source zones 110, whereinthe diode regions 116 are p-doped and the drift zone 121 is n-doped.According to another embodiment, the transistor cells TC are p-channelFET cells with n-doped body zones 115 and p-doped source zones 110,wherein the diode regions 116 are n-doped and the drift zone 121 isp-doped.

When a potential at the gate electrode 155 exceeds or falls below athreshold voltage of the semiconductor device 500, minority chargecarriers in the body zones 115 form inversion channels connecting thesource zones 110 with the drift structure 120, thereby turning on thesemiconductor device 500. In the on-state, a load current flows throughthe semiconductor body 100 approximately along the vertical directionbetween the first and second load electrodes 310, 320.

As illustrated in FIGS. 1A and 1C the first surface 101 may be aserrated surface with long flat first surface sections 101 a that are(0001) crystal planes and short, steep second surface sections 101 btilted to the first surface sections 101 a by an angle greater 0 degree,e.g., about 90 degree and connecting the first surface sections 101 a.The second surface sections 101 b may be <1-100> crystal planes. Steps108 formed along the edges between first and second surface sections 101a, 101 b run parallel to the <11-20> crystal direction as illustrated inFIG. 1A.

According to another embodiment, the first surface 101 may be planar andtilted to a main crystal plane, e.g., the (0001) crystal plane by anoff-axis angle of at least 2 degree, e.g., about 4 degree or at least −2degree, e.g., about −4 degree. In this case, a mean surface plane 101 xdefined by the least squares mean plane coincides with the first surface101.

The serrated first surface 101 results from cutting a crystal ingot atan off-axis angle α with respect to the <1-100> crystal direction. Theoff-axis angle α between the cut plane and the <1-100> crystal directionmay be any angle in a range from about 2 to 12 degree or −2 to −12degree, for example in a range from 3 to 8 degree or −3 to −8 degree.According to an embodiment the off-axis angle α is approximately 4degree or −4 degree. The cut plane may coincide with or is parallel to amean surface plane 101 x of the first surface 101. In case ofequidistant steps 108, the mean surface plane 101 x, which is defined bythe least square mean plane, cuts the first and second surface sections101 a, 101 b at the half step size.

A serrated substrate surface facilitates step controlled epitaxy duringwhich silicon and carbon atoms impinge on the serrated substrate surfaceand start ordered crystal growth at the steps. A top surface of anepitaxial layer grown on the serrated substrate surface of a 4H—SiCwafer used as substrate for the epitaxy images the serrated substratesurface of the 4H—SiC wafer substrate. A chemical-mechanical polishingmay planarize the serrated surface and a passivation layer, e.g., acarbon layer, may suppress redeposition of the silicon and carbon atomsalong main crystal planes during following heating processes.

With the mean surface plane 101 x of the semiconductor body 100 tiltedagainst the <1-100> crystal direction instead tilted against the <11-20>crystal direction and with the trench gate structures 150 extendingoriented along the <1-100> crystal direction, the longitudinal axes ofthe trench gate structures 150 are parallel to a vertical projection ofthe <1-100> crystal direction onto the mean surface plane 101 x anddeviate from the vertical projection of the <1-100> crystal direction bynot more than 5 degree or at most 2 degree. The sidewalls of the trenchgate structures 150, which run orthogonal to the steps 108, are (11-20)and (−1-120) crystal planes and deviate from a normal to the meansurface plane 101 x, i.e., the vertical direction, by not more than 5degree. According to an embodiment, the sidewalls extend vertically tothe mean surface plane 101 x as shown in FIG. 1B.

Since along both sidewalls the charge carrier mobility is the same,symmetric transistor cell layouts may use both sides of the trench gatestructures 150 for forming MOS gated channels through the body zones115. Even if process fluctuations result in slightly tapered trench gatestructures 150 with sidewalls deviating by up to 5 degree from thevertical direction, the resulting angular misalignment between thesidewalls of the transistor mesas 170 and the (11-20), (−1-120) crystalplanes is approximately equal such that on both sides of the trench gatestructures 150 electron mobility and threshold voltage are the same. Asa consequence, in case the transistor cells TC are turned on a currentdistribution is uniform across the semiconductor body 100.

FIG. 2 shows the crystal planes and crystal directions in a lattice cellof a hexagonal crystal of 4H—SiC with the principal <0001> crystaldirection perpendicular to the (0001) basal plane or C-plane. Crystalplanes with high charge carrier mobility are the {1-100} crystal planesor M-planes and the {11-20} crystal planes or A-planes, wherein thecharge carrier mobility in the {11-20} crystal planes may be about 20%higher than in the {1-100} crystal planes. The A-planes are members of aset of planes including the (11-20), (1-210), (−2110), (2-1-10),(−12-10) and (−1-120) crystal planes and the following descriptionrefers to the (11-20) crystal plane as an arbitrary one of the A-planes.

FIGS. 3A and 3B refer to conventional silicon carbide devices.Typically, a surface of a SiC semiconductor device has an off-axis angleα of 4 degree with respect to the <11-20> crystal direction.

For example, a 4H—SiC crystal ingot is cut at an off-axis angle in arange from 2 degree to 8 degree with respect to the <11-20> crystaldirection to obtain a wafer substrate. During a thermal treatment of thewafer substrate, silicon and carbon atoms rearrange along the crystaldirections such that a serrated surface is formed with long flat firstsurface sections parallel to the <11-20> crystal direction. Short steepsecond surface sections, which may be (11-20) crystal planes, connectthe first surface sections with each other. When an epitaxial layer isgrown on the serrated wafer surface by step-controlled epitaxy, theedges at the steps between first and second surface sections order theimpinging silicon and carbon atoms in a way such that the grownepitaxial layer continues the 4H—SiC crystal structure.

FIG. 3A shows the resulting orientation of a lattice cell in asemiconductor body obtained from the process described above. The <0001>crystal direction declines into the direction of the (11-20) crystalplanes and both are tilted with respect to a horizontal line.

FIG. 3B shows a trench gate structure 150 that extends from a front sideinto a semiconductor body 100. A serrated first surface 101 a of thesemiconductor body 100 includes long flat first surface sections 101 aparallel to the <11-20> crystal direction. Short steep second surfacesections 101 b, which may be (11-20) crystal planes, connect the firstsurface sections 101 a with each other. The steps in the first surface101 may be approximately symmetric with respect to a mean surface plane101 x, which is tilted to the <11-20> crystal direction at the off-axisangle α.

The trench gate structure 150 tapers with increasing distance to the topsurface 101 a at a taper angle β with respect to the vertical direction.If the taper angle β and the off-axis angle α are equal, a firstsidewall 104 of the trench gate structure 150 is a (11-20) crystalplane, whereas an opposite second sidewall 105 is tilted to the (11-20)crystal planes by an angular misalignment γ=α+β. Since electron mobilityand threshold voltage strongly depend on the crystal orientation, aninversion channel formed along the second sidewall 105 is significantlyless effective than an inversion channel formed along the first sidewall104.

In addition, the first and second sidewalls 104, 105 are only smooth andwithout steps if the horizontal direction along which they extend isperfectly parallel to the (11-20) crystal planes. At a slight angularmisalignment between the crystal planes and the longitudinal axes of thetrench gate structure 150, the first and second sidewalls 104, 105 cutthe (11-20) crystal planes. A high temperature anneal performed afterformation of trench may form steps compensating the deviation of thelongitudinal axes of the trench from the (11-20) crystal planes. Thesteps in turn may locally change the characteristics of the inversionchannels and may degrade device reliability.

FIGS. 4A to 4C illustrate the orientation of trench gate structures withrespect to the crystal directions according to the embodiments. Insteadof cutting a wafer with an off-axis angle α tilted to the <11-20>crystal direction, the wafer is cut with an off-axis angle tilted to the<1-100> crystal direction which is orthogonal to the <11-20> crystaldirection. In other words, while conventionally the <0001> crystaldirection declines in direction of the (11-20) crystal planes, theembodiments are based on a crystal with the <0001> crystal directiondeclining into direction of the <1-100> crystal planes.

FIG. 4A shows the resulting orientation of a lattice cell in asemiconductor body obtained from the process described above. Thelattice cell is tilted around the tilt axis 109 towards the viewer suchthat the <0001> crystal direction as well as the (1-100) crystal planesare tilted outwardly of the drawing plane into the direction of theviewer. The orientation of the (11-20) crystal planes remains orthogonalto the drawing plane.

As illustrated in FIGS. 4B and 4C, the semiconductor body 100 withtrench gate structures 150 and transistor mesas 170 as illustrated inFIGS. 1A to 1D is obtained from a wafer of a single crystalline materialhaving a hexagonal crystal structure, wherein a mean surface plane 101 xof a serrated first surface 101 has an off-axis angle α with respect tothe <1-100> crystal direction. The trench gate structures 150 extendalong the <1-100> crystal direction such that vertical sidewalls of thetransistor mesas 170 are {11-20} crystal planes, e.g., (11-20) and(−1-120) crystal planes. Both sidewalls 104, 105 have identical surfaceproperties such that both trench gate sidewalls 104, 105 are identicalwith respect to charge carrier mobility. Along both sidewalls 104, 105 acurrent density is equal and overall current distribution is moreuniform. Even at a minor taper of the trench gate structures 105, theeffect of the tapering is the same on both sidewalls 104, 105 such thata uniform current distribution can be achieved even when processfluctuations cause slightly tapered sidewalls 104, 105.

The semiconductor device 500 of FIGS. 5A and 5B is a silicon carbideIGFET based on the semiconductor device 500 of FIGS. 1A to 1D, whereinthe first load electrode 310 forms or is electrically connected orcoupled to a source terminal S and the second load electrode 320 formsor is electrically connected to a drain terminal D.

In FIG. 5A the principal <0001> crystal direction is slightly tilted tothe perpendicular on the drawing plane. The <1-100> crystal directionruns parallel to the trench gate structures 150 and is slightly tiltedto the drawing plane by the off-axis angle α. Vertical sidewalls of thetransistor mesas 170 are {11-20} crystal planes like (11-20) and(−1-120) crystal planes.

In FIG. 5B the <0001> crystal direction is tilted by the off-axis angleα to the drawing plane and the <1-100> crystal direction is tilted bythe off-axis angle α to the perpendicular onto the drawing plane.

The trench gate structures 150 are stripes and may orthogonallyintersect steps that may be formed between staggered surface sections atthe first surface 101. According to another embodiment, the firstsurface 101 may be planar. The trench gate structures 150 may be equallyspaced from each other, may have uniform width and may extend from oneside of a transistor cell area to the opposite side. Mesa contactstructures 305 extend from the first surface 101 into the transistormesas 170. According to the illustrated embodiment the mesa contactstructures 305 have a greater vertical extension than the trench gatestructures 150 and directly adjoin diode regions 116 formed at least inthe vertical projection of the transistor mesas 170 between thetransistor mesas 170 and the second surface 102. The diode regions 116form third pn junctions pn3 with the drift structure 120 and provide afly back diode functionality integrated in the semiconductor device 500.Portions of the diode regions 116 may overlap with the verticalprojection of the trench gate structures 150 and may shield activeportions of the gate dielectric 151 against the high potential of thesecond load electrode 320 in the blocking state of the semiconductordevice 500.

The drift structure 120 may include current spread zones 122 which maydirectly adjoin the body zones 115. The current spread zones 122 mayextend between neighboring diode regions 116. Unipolar homojunctionsbetween the current spread zones 122 and the drift zone 121 may have agreater distance to the first surface 101 than the third pn junctionspn3 formed between the diode regions 116 and the drift zone 121. A meannet dopant concentration in the current spread zones 122 is at least tentimes as high as a mean net dopant concentration in the drift zone 121.Portions of the current spread zones 122 may overlap with the verticalprojection of the diode regions 116 and may extend between neighboringdiode regions 116.

The mesa contact structure 305 splits the transistor mesa 170 in twoportions on both sides of the intermediate mesa contact structure 305,wherein each portion may include the semiconducting portion of atransistor cell TC. According to other embodiments, the verticalextension of the mesa contact structures 305 may be smaller than thevertical extension of the trench gate structures 150 such that the mesacontact structures 305 do not completely partition the body zones 115.Then, a heavily doped contact region of the conductivity type of thediode region 116 may be formed in the vertical projection of the mesacontact structure 305 to electrically connect the concerned diode region116 through the mesa contact structure 305 with the first load electrode310.

An interlayer dielectric 210 sandwiched between the first load electrode310 and the gate electrode 155 dielectrically insulates the first loadelectrode 310 from the gate electrode 155. The interlayer dielectric 210may include one or more dielectric layers from silicon oxide, siliconnitride, silicon oxynitride, doped or undoped silicate glass, forexample BSG (boron silicate glass), PSG (phosphorus silicate glass) orBPSG (boron phosphorus silicate glass), by way of example. For furtherdetails reference is made to the description of the previous figures.

In FIG. 6A, the crystal directions are the same as in FIG. 5A and inFIG. 6B the same as in FIG. 5B.

Deep trench contact structures 306 extend from the first load electrode310 through the trench gate structures 150 and directly adjoin dioderegions 116 formed in the vertical projection of the trench gatestructures 150. The trench contact structures 306 partition the trenchgate structures 150 into spacer portions on opposite sides of theintermediate trench gate structure 306. Contact insulator structures 220are sandwiched between the trench contact structures 306 and adjoiningspacer portions 155 a, 155 b of the gate electrode 155. In addition,mesa contact structures 305 extend from the first surface into thetransistor mesas 170 and are electrically connected to the source zones110 and, through heavily doped contact zones 115a, to the body zones 115

FIGS. 7A to 7D illustrate a semiconductor device 500 with shieldingportions 116 b of diode regions 116 forming a layer with point-symmetricopenings centered to transistor mesas 170 which alternate with diodemesas 180 along at least the first horizontal direction.

The orientation of the crystal directions in FIG. 7A is the same as inFIGS. 5A and 6A and is in FIG. 7B the same as in FIGS. 5B and 6B.

In addition to the transistor mesas 170, the semiconductor body 100includes diode mesas 180, wherein each diode mesa includes at least aportion of a diode region 116. The diode region 116 of a diode mesa 180extends from one of the neighboring trench gate structure 150 to theopposite one. The diode regions 116 may include portions outside themesa portions formed between neighboring trench gate structures 150,wherein a vertical extension of the diode regions 116 is greater than avertical extension of the trench gate structures 150. The diode regions116 may vertically overlap with the trench gate structures 150 such thatshielding portions 116 b of the diode regions 116 are formed in thevertical projection of the trench gate structures 150 and are spacedfrom the transistor mesas 170 along the horizontal direction. In thetransistor mesas 170 both the source zones 110 and the body zones 115may extend from one neighboring trench gate structure 150 to theopposite trench gate structure 150.

The transistor mesas 170 and the diode mesas 180 alternate along thefirst horizontal direction or along both the first horizontal directionand the second horizontal direction as illustrated. Neighboringtransistor and diode mesas 170, 180 directly adjoin to each other alongthe first horizontal direction and are separated from each other byintermediate trench gate structures 150 along the second horizontaldirection.

The shielding portions 116 b of the diode regions 116 may form acontiguous layer between the bottom of the trench gate structures 150and the second surface 102. The contiguous layer has openings 117centered to the transistor mesas 170. A width z1 of the openings 117orthogonal to the first horizontal direction may be equal to a width z2of the openings 117 along the first horizontal direction.

In the blocking mode depletion zones extend from all four horizontaldirections into the direction of the transistor mesas 170 andeffectively shield the active portions of the gate dielectric 151against a high electric field caused by the blocking voltage. Theopenings may be point-symmetric, for examples squares, octagons or otherregular polygons, or circles. As regards further details, reference ismade to the description of the previous Figures.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat a variety of alternate and/or equivalent implementations may besubstituted for the specific embodiments shown and described withoutdeparting from the scope of the present invention. This application isintended to cover any adaptations or variations of the specificembodiments discussed herein. Therefore, it is intended that thisinvention be limited only by the claims and the equivalents thereof.

What is claimed is:
 1. A semiconductor device comprising: a plurality oftrench gate structures in a semiconductor body with hexagonal crystallattice, wherein a mean surface plane of a first surface is tilted withrespect to a <1-100>crystal direction by an off-axis angle, wherein anabsolute value of the off-axis angle is in a range from 2 degrees to 12degrees and trench sidewalls of the plurality of trench gate structuresare aligned with the (11-20) crystal plane; and a plurality oftransistor mesas formed from portions of the semiconductor body betweenneighboring trench gate structures of the plurality of trench gatestructures, wherein sidewalls of the plurality of transistor mesasdeviate from a normal to the mean surface plane by not more than 5degrees.
 2. The semiconductor device of claim 1, wherein the firstsurface is a serrated surface that comprises first surface sectionsformed by (1-100) crystal planes and second surface sections that aretilted with respect to the first sections and that connect the firstsurface sections with each other.
 3. The semiconductor device of claim1, wherein the off-axis angle is in a range from 3 to 8 degrees or from−3 to −8 degrees.
 4. The semiconductor device of claim 1, wherein theplurality of trench gate structures are stripe-shaped with alongitudinal axis parallel to a projection of the <1-100> crystaldirection on the mean surface plane.
 5. The semiconductor device ofclaim 1, wherein the semiconductor body comprises silicon carbide of the4H polytype.
 6. The semiconductor device of claim 1, wherein theplurality of transistor mesas comprise body zones forming first pnjunctions with a drift structure and second pn junctions with sourcezones, the source zones formed between the body zones and the firstsurface.
 7. The semiconductor device of claim 1, further comprising:diode regions forming third pn junctions with the drift structure andoverlapping with neighboring trench gate structures of the plurality oftrench gate structures in a vertical projection orthogonal to the meansurface plane.
 8. The semiconductor device of claim 1, furthercomprising mesa contact structures extending from the mean surface planeinto the plurality of transistor mesas and directly adjoining the sourcezones and the body zones.
 9. The semiconductor device of claim 8,wherein a vertical extension of the mesa contact structures in thesemiconductor body is equal to or greater than a vertical extension ofthe plurality of trench gate structures.
 10. The semiconductor device ofclaim 7, further comprising trench contact structures extending from thefirst surface through the plurality of trench gate structures anddirectly adjoining the diode regions.
 11. The semiconductor device ofclaim 10, wherein a vertical extension of the trench contact structuresis equal to or greater than a vertical extension of the plurality oftrench gate structures.
 12. The semiconductor device of claim10, whereineach of the plurality of trench gate structures include first spacerportions and second spacer portions of a gate electrode on oppositesides of the respective trench contact structures.
 13. The semiconductordevice of claim 10, further comprising mesa contact structures extendingfrom the mean surface plane into the plurality of transistor mesas anddirectly adjoining the source zones and the body zones.
 14. Thesemiconductor device of claim 1, further comprising diode mesascomprising at least portions of diode regions that form third pnjunctions with the drift structure and that directly adjoin twoneighboring trench gate structures of the plurality of trench gatestructures, respectively, wherein transistor and diode mesas alternateat least along a first horizontal direction parallel to a longitudinalaxis of the plurality of trench gate structures.
 15. The semiconductordevice of claim 14, wherein a distance between the first surface and thethird pn junctions is greater than a vertical extension of the pluralityof trench gate structures perpendicular to the first surface.
 16. Thesemiconductor device of claim 14, wherein the diode regions includeshielding portions in a vertical projection of the plurality of trenchgate structures.
 17. The semiconductor device of claim 16, wherein theshielding portions do not overlap with the plurality of transistormesas.
 18. The semiconductor device of claim 14, wherein the transistorand diode mesas further alternate, separated by trench gate structuresof the plurality of trench gate structures, along a second horizontaldirection orthogonal to the first horizontal direction.
 19. Thesemiconductor device of claim 1, wherein the drift structure comprises alightly doped drift zone and current spread zones between the drift zoneand the body zones, and a mean net dopant concentration in the currentspread zones is at least twice as high as a mean net dopantconcentration in the drift zone.
 20. The semiconductor device of claim1, wherein the sidewalls of the plurality of transistor mesas arealigned with the <1-100> crystal direction.
 21. The semiconductor deviceof claim 1, wherein the first surface is planar.
 22. The semiconductordevice of claim 1, wherein the trench sidewalls of the plurality oftrench gate structures are substantially perpendicular to the firstsurface.
 23. A semiconductor device comprising: a plurality of trenchgate structures in a semiconductor body with hexagonal crystal lattice,wherein a mean surface plane of a first surface is tilted with respectto a <1-100> crystal direction by an off-axis angle and trench sidewallsof the plurality of trench gate structures are aligned with the (11-20)crystal plane; and a plurality of transistor mesas formed from portionsof the semiconductor body between neighboring trench gate structures ofthe plurality of trench gate structures, wherein sidewalls of theplurality of transistor mesas deviate from a normal to the mean surfaceplane by not more than 5 degrees.
 24. A semiconductor device comprising:a plurality of trench gate structures in a semiconductor body withhexagonal crystal lattice, wherein a mean surface plane of a firstsurface is tilted with respect to a <1-100> crystal direction by anoff-axis angle; and a plurality of transistor mesas formed from portionsof the semiconductor body between neighboring trench gate structures ofthe plurality of trench gate structures, wherein sidewalls of theplurality of transistor mesas are aligned with the (11-20) crystal planeand deviate from a normal to the mean surface plane by not more than 5degrees.